Types of Lime: Read the Binder’s Form and Classification
Compare quicklime, hydrated lime, lime putty, NHL and blended limes. Read the labels and choose a binder compatible with existing brick or stone.
The most useful distinction between types of lime is how they harden: non-hydraulic—or air—lime hardens by carbonation; hydraulic lime also hardens through reactions with water. Quicklime, hydrated lime and lime putty describe different stages or forms of the material, not three interchangeable mortar specifications.
For historic repairs, start with the wall rather than the bag. The National Park Service recommends replacement pointing mortar that is softer and more vapor-permeable than the masonry units, and no harder or less vapor-permeable than the historic mortar. A stronger binder is not automatically a better repair: an excessively hard or impermeable joint can shift stress and moisture-related damage into the brick or stone. NPS Preservation Brief 2 explains these compatibility requirements.
The main types and forms
| Name | What it means | Practical consequence |
|---|---|---|
| Quicklime | Unslaked lime, principally in oxide form, produced by burning limestone | Requires controlled slaking; used to make putty or specified hot-mixed mortars, not as a direct substitute for hydrated lime |
| Hydrated lime | Lime already slaked with water; commonly sold as dry powder | Avoids on-site quicklime slaking, but the product designation must still be checked |
| Lime putty | Slaked lime supplied as a wet paste | A form of hydrated lime, used for limewash, fine plaster and compatible mortar or render specifications |
| Natural hydraulic lime—NHL | Lime whose hydraulic properties come from the limestone’s natural composition | Sets through water reactions as well as carbonation; normally supplied as powder |
| Formulated lime—FL | Air lime and/or NHL with added hydraulic or pozzolanic material | Composition matters; check declared additions, including any cement |
| Hydraulic lime—HL | A lime binder blended with other materials, potentially including cement, slag, fly ash or limestone filler | The HL designation is distinct from NHL; assess the actual blend before specifying it |
The European classifications follow EN 459-1:2015’s building-lime definitions. In that standard, hydrated lime names air lime in powder, putty or slurry form. In broader trade usage, hydraulic lime may also be described as a bagged hydrate, so “hydrated” alone does not settle its setting behavior. SPAB lime guidance
Non-hydraulic lime: carbonation, not simply drying
Non-hydraulic lime reacts with carbon dioxide to form calcium carbonate. Rapid drying is not a shortcut to a sound joint: NPS warns that drying lime mortar too quickly reduces carbonation, adhesion and durability. Preservation Brief 2
Putty and dry hydrated non-hydraulic lime belong to this same broad family, but they need not handle identically. The Society for the Protection of Ancient Buildings prefers non-hydraulic putty for its workability, plasticity and binding properties. It also cautions that hydraulic lime normally cannot be stored under water as putty because it will set. Check the binder identity before soaking a bag of lime; do not assume it will produce a suitable non-hydraulic putty. SPAB lime guidance
Hydraulic lime: an additional water-driven set
Burning limestone containing reactive silica and aluminium compounds produces hydraulic components. These react with water to form hardened compounds, while the remaining lime also carbonates. This can provide useful early durability where exposure makes a slow-setting air-lime mortar difficult to protect.
The trade-off is not merely speed. SPAB explains that increasing hydraulicity generally brings greater strength but lower permeability and flexibility. A weak hydraulic lime—or an appropriately specified air lime with a pozzolan, an addition that reacts with lime to provide a hydraulic set—may suit moderately exposed external pointing or render. This is not a universal prescription for exterior walls. SPAB lime guidance and Lime: The Basics explain these distinctions.
What NHL 2, 3.5 and 5 actually tell you
The NHL numbers identify classes based on 28-day compressive strength under standardized testing. They are not mix ratios, and they do not predict the exact strength of your finished pointing mortar. EN 459-1:2015
Historic England warns that the permissible properties overlap, products within one class can vary widely, and the tests do not establish long-term performance. Consequently, “NHL 3.5 for every exterior wall” is not a defensible specification. Ask for product-specific mortar data and consider the aggregate, proportions, exposure and curing—not just the number. Historic England’s NHL research
Read the other letters carefully
Under the European classification, CL means calcium lime and DL means dolomitic lime. Both are air-lime families; DL contains calcium and magnesium compounds. A label such as CL 90 is a composition classification, not a mortar-strength grade comparable to NHL 2. EN 459-1:2015
In US terminology, Type S hydrated lime is not Type S mortar. ASTM C207 identifies the lime ingredient; ASTM C270 identifies mortar types. The same Type S lime can be used in different cement–lime mortar proportions. National Lime Association fact sheet
What to establish before buying
For conservation work, identify sound original mortar rather than copying a later hard repair. Check the brick or stone, moisture sources and exposure; then agree a complete mortar specification and a representative test panel. Laboratory analysis can help identify ingredients, but NPS cautions against basing the replacement specification solely on analysis. Preservation Brief 2 provides the assessment guidance; for the repair sequence, see masonry repointing: diagnose before repair.
For new construction, select the binder within the project’s structural and exposure requirements rather than transferring a heritage pointing recipe unchanged.
If the specification involves quicklime, treat slaking as a specialist operation. Quicklime releases heat on contact with water and can harm eyes, skin and the respiratory system; follow the product safety data and a controlled working method. NIOSH calcium oxide guidance covers the hazards, while our quicklime explainer distinguishes slaking from ordinary mortar mixing.